Shaun o said:
Moricon is also right with what he has said in his post,
It could be as simple as changing a setting to pci-e slot as your primary graphics card in the bios.
I would also check that you do in fact have the extra power feed from your psu = 6 pin pci-e power connector on your GT 250 graphics card, see if it is loose.
With out it in it is also likely to be the cause of black screens, just before it goes to load into windows 7, and to the desktop.
The reason for it is windows xp, infact used a 2d driver for its desktop display of windows xp.
Windows 7 uses a 3d driver to display the desktop. in 3d mode the GT 250 card will need more power to run.
All i can hope is this was a bit more help, than from MS.
And that you now have a good idea of how to fix this problem.
Let me know how you got on with it.
dito on what shaun said.